Special-Purpose Computer for Molecular Dynamics
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The real situation is more complicated because other impurity atoms will be present, particularly oxygen which is bonded or anchored to two silicon neighbors but is located in the spaces or interstices between the silicon atoms. The oxygen strains the crystal in the opposite sense from carbon, and the two types of impurity prefer to occupy adjacent sites as this leads to an overall reduction in the strain. To form such pairs the carbon impurity would have to jump from one lattice site to the next, but this is not easily achieved because the atoms of the crystal are in the way and an exchange process must occur. On the other hand, for atoms in interstitial sites where the bonding is weak, migration through the crystal can proceed much more readily. When silicon crystals are bombarded with nuclear particles, collisions occur and some
atoms are knocked off their lattice sites into the interstices. Carbon impurity atoms are very readily transferred during this process and can then migrate even at room temperature. They are then trapped at other imperfections in the crystal including undisplaced carbon and oxygen atoms to form a whole series of complicated clusters of impurities. It is crucially important to understand the structure and nature of such defects because they can have detrimental effects on the properties of silicon used to make (a) integrated electronic circuits or (b) devices for power circuits. In this paper, we review the current state of knowledge of these effects in silicon immediately after its growth, and following various processing, including high-energy bombardment and heat treatments at elevated temperatures which can also lead to displacement of carbon atoms from lattice
sites. The processes are monitored mainly by measurements of transparency of the crystals with optical equipment operating in the infrared spectral region. This diagnostic method can be understood by comparison with other materials that are transparent to visible light. The presence of defects or impurities in such a material, including glass, removes light at certain wavelengths and the material acquires a characteristic color. The view of silicon in the infrared is similarly "colored" by the defects involving carbon and the color changes produced by processing tell us which types of defect have been produced or destroyed. Colors characteristic of several defects have been identified but considerbly more research effort will be needed before a full understanding can emerge.
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Special-Purpose Computer for Molecular Dynamics H. R. Carleton State University of New York, Stony Brook, Long Island, NY 11794, U.S.A. Physical systems have traditionally been analyzed by searching for functional relationships between various measurable parts of the system such as the density, temperature, particle forces, etc. These functional relationships are represented by complex mathematical functions which are then analyzed by classical methods for the solution of the integro-differential equations. Analytic solutions thus obtained can t
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